1. Field of the Invention
The present invention relates to an image reading apparatus that illuminates documents and reads the images of the documents by a photoelectric conversion unit, and particularly relates to an image forming unit that forms images on the photoelectric conversion unit.
2. Description of the Related Art
Hitherto, an image scanner for reading image data on documents has been known as an image reading apparatus, in Japanese Patent Laid-Open No. 3-113961, for example.
In such an image scanner, an image forming lens and a line sensor are fixed, and exposure scanning of a document is carried out by moving only reflecting mirrors, thus reading image data.
Recently, to simplify the structure of such an apparatus, carriage-integrated scanning systems that integrate mirrors, an image forming lens, and a line sensor, have been adopted to scan documents.
FIG. 5 shows an image reading apparatus of a known carriage-integrated scanning system. A light beam emitted by a light source 501 illuminates a document 503 mounted on a document table glass 502, then the reflected light beam from the document 503 gets bent along an optical path inside a carriage 511 through first to third mirrors 504 to 506, and thus an image forming lens 509 forms an image onto a line sensor 510 as the light beam passes through the image forming lens 509. The carriage 511 is moved by a driving motor (not shown) in the direction arrow A (feed direction) to read the image data of the document 503. The image forming lens 509 is fixed in a lens barrel 512, and the first to third mirrors 504 to 506 and the lens barrel 512 are fixed to the carriage 511.
In an image reading apparatus such as a digital copy machine, however, since high resolution and high speed are required, the field angle must be narrow to improve the resolution of the image forming lens, but then the length of the optical path is extended and the light quantity is decreased. If the optical path length is shortened, the field angle becomes wider, which lowers the resolution and thus makes it difficult to employ a carriage-integrated scanning system.
An image reading apparatus of a carriage-integrated scanning system adopting an off-axis optical system that forms an image by the reflection of a light beam, as disclosed in Japanese Patent Laid-Open No. 8-292371 or the like, is presented in Japanese Patent Laid-Open No. 2002-335375 (US Patent Application No. 20030038228). With respect to a reference axis along a light beam going through the image center and the pupil center, an off-axis optical system is defined as an optical system that includes a curved surface (off-axis curved surface) whose surface normal at the intersection point of the surface and the reference axis is not on the reference axis. In Japanese Patent Laid-Open No. 2002-335375, a construction that avoids chromatic aberration, with air as the medium between the reflecting surfaces of an off-axis optical system, particularly for scanning color documents, is presented.